a) Field of the Invention
The present invention relates to a method and apparatus for severing disks of brittle material, such as wafers.
b) Description of the Related Art
It is known to sever disks made of a brittle material such as glass, ceramic, silicon, quartz, gallium arsenide or sapphire by means of a laser through induced thermal stresses.
For this purpose it is essential that the severing edge is formed exactly along the planned lines (referred to hereinafter as the precision of the severing line) and that the stress field caused by a sufficiently high temperature gradient is localized as precisely as possible on the planned severing edge.
DE 693 04 194 T2 discloses a method of the type mentioned above in which a beam bundle is moved on the surface along a desired severing line relative to the workpiece followed by a jet of coolant (water-air mixture) which is directed in a narrowly localized manner on a point of the heated severing line at a distance from the point of impingement of the beam bundle.
As is fundamental in all of the methods in which stresses are induced by means of laser radiation leading to the formation of cracks, the energy of the beam bundle is controlled in such a way that while taking into account the relevant method parameters and workpiece parameters the volume in the surface region of the workpiece is heated to a temperature below the softening point of the material in order to generate high compressive stresses in the volume.
In order to achieve a high energy input at high speed, it is suggested that the beam spot of the beam bundle is elliptical, which already shows the realization that a significant narrowing of the heating zone transverse to the cutting direction increases the cutting precision.
At the same time, it was realized that a minimum width of the beam spot is necessary to cause the required compressive stresses due to the limited action period and the length of the beam spot.
The optimal value indicated for the transverse dimension is 0.2 times to 2.0 times the thickness of the material.
The cross section of the coolant jet is not mentioned in DE 693 04 194 T2. According to the invention, a coolant jet of this kind has a diameter of 1 to 2 mm2.
According to an embodiment example in DE 693 04 194 T2, the method described therein can be used to cut disks having a diameter of 31.2 mm, e.g., from sheet glass having a thickness of 1.2 mm.
Practical trials have shown that the precision of the severing edges that can be achieved with this method when severing a disk in very narrow strips or chips, e.g., less than 500 μm, is insufficient.
The extract from WO 03/018276 A1 likewise discloses a method for severing brittle material in which a laser beam is moved relatively along a planned severing line on the surface of a workpiece. At the same time, the entire surface is covered by a coolant film in which a hole is generated at the respective point of impingement of the laser beam by means of a gas flow. The spray position of the gas must likewise be moved along the severing line corresponding to the laser beam.
In contrast to DE 693 04 194 T2, tensile stresses are initially generated at the surface, not in a localized manner but so as to be homogeneously distributed over the entire surface of the workpiece, by cooling the entire surface during the laser irradiation. The severing, crack is formed immediately by the application of laser radiation and not after a delay caused by the interval between the impingement of the beam bundle and the coolant jet as in DE 693 04 194 T2. Consequently, the zone of the occurring compressive stresses caused by the introduction of energy by laser radiation is narrower than that which can be achieved by a method corresponding to the first reference cited above. Since no time is given for heat conduction, the occurring temperature gradient is also not only more sharply concentrated on the severing line, but is also higher. The precision of the cut edges that can be achieved is therefore improved, which is especially significant when cutting into very small pieces.
A method according to WO 03/018276 is disadvantageous in that it also compulsorily requires a liquid coolant. Compared to a severing method in which material is removed by means of laser, a severing method based on the introduction of thermal stresses has the advantage that basically no waste products occur so that there is no soiling of the workpiece surface. Naturally, this advantage is neutralized when a liquid coolant comes into contact with the workpiece surface as in the two methods described above.
In terms of apparatus, not only are technical means required in addition to apply, hold and remove the coolant film, but means are also required for generating and guiding a flow of gas over the workpiece.
Compared to a method according to DE 693 04 194 T2, a method according to WO 03/018276 also has the disadvantage that the tensile stresses and the compressive stresses only form at the surface and therefore the severing crack frequently terminates as a blind crack in the volume of the material.